The landscape inland of St Austell in Cornwall bears the indelible mark of nearly 300 years of mining activity.
Home to the largest china clay deposits in the world, this dramatic terrain is dominated by the ‘Cornish Alps’ which are formed from around 600 million tonnes of spoil material.
One of the area’s largest mica dams is the site for CleanEarth’s latest solar farm development, and the six acre site’s industrial provenance presents some interesting geotechnical challenges.
As CleanEarth’s operations director, Tristan Grimes, says, “you don’t find many man-made landscapes on this scale, and we’ve had to get our heads around a few unusual features on this project.”
Flood prevention, subsidence risk, topsoil protection
The fact that it’s on a ‘mica dam’ immediately suggests that water may be an issue, and extensive work was needed on drainage to mitigate the winter flooding that the site is prone to. In addition, all equipment is being raised half a metre above ground level to keep it clear of any flood waters.
“We are building in protection against a 1 in 100 year flooding event,” says Tristan, “as water and electricity are best kept apart from each other.”
Piling into spoil material also takes particular care, with a higher number of test piles being deployed and a three week wait before checking for settlement.
The subsidence risk is greater if dusty topsoil is being constantly eroded by water and weathering, so the site has been hydro-seeded to promote the growth of vegetation to hold the topsoil together.
Working to Quarries Regulations
Although the Lower Ninestones site is not currently operational, there is still active china clay quarrying in the area, so the project needs to adhere to the regulations for health and safety at quarries. This means extra protection is needed for all cabling, with high voltage runs in steel wire armoured cable and the low voltage DC cabling in protective ducting.
“We’ve dug several miles of trenches,” says Tristan, “including a 2.5 km stretch from the inverter station to the 33 kV grid connection – which I think is the longest we’ve done on any solar PV installation.”
On schedule for November completion
Dry weather over the summer and early autumn has meant few interruptions to the project and everything remains on track for completion in early November.
Once commissioned, all electricity generated by the 2.5 MW solar farm will be exported direct to the grid, providing enough energy to power more than 450 Cornish homes.
The Lower Ninestones project adds to more than a dozen solar farms that CleanEarth have completed in Cornwall, and more than 500 solar PV installations of all sizes across the UK.
CleanEarth is working with Centrica plc, alongside the Cornwall Local Energy Market (LEM) to realise this project. The LEM is part-funded by the European Regional Development Fund under the European Structural and Investment Funds Programme 2014-2020.